Re: [PATCH v8 06/15] x86: Add early SHA support for Secure Launch early measurements

ross.philipson
2024-04-04T04:56:47Z

On 4/3/24 4:56 PM, Eric Biggers wrote:
> On Wed, Apr 03, 2024 at 09:32:02AM -0700, Andy Lutomirski wrote:
>> On Fri, Feb 23, 2024, at 10:30 AM, Eric Biggers wrote:
>>> On Fri, Feb 23, 2024 at 06:20:27PM +0000, Andrew Cooper wrote:
>>>> On 23/02/2024 5:54 pm, Eric Biggers wrote:
>>>>> On Fri, Feb 23, 2024 at 04:42:11PM +0000, Andrew Cooper wrote:
>>>>>> Yes, and I agree.  We're not looking to try and force this in with
>>>>>> underhand tactics.
>>>>>>
>>>>>> But a blind "nack to any SHA-1" is similarly damaging in the opposite
>>>>>> direction.
>>>>>>
>>>>> Well, reviewers have said they'd prefer that SHA-1 not be included and given
>>>>> some thoughtful reasons for that. But also they've given suggestions on how to
>>>>> make the SHA-1 support more palatable, such as splitting it into a separate
>>>>> patch and giving it a proper justification.
>>>>>
>>>>> All suggestions have been ignored.
>>>>
>>>> The public record demonstrates otherwise.
>>>>
>>>> But are you saying that you'd be happy if the commit message read
>>>> something more like:
>>>>
>>>> ---8<---
>>>> For better or worse, Secure Launch needs SHA-1 and SHA-256.
>>>>
>>>> The choice of hashes used lie with the platform firmware, not with
>>>> software, and is often outside of the users control.
>>>>
>>>> Even if we'd prefer to use SHA-256-only, if firmware elected to start us
>>>> with the SHA-1 and SHA-256 backs active, we still need SHA-1 to parse
>>>> the TPM event log thus far, and deliberately cap the SHA-1 PCRs in order
>>>> to safely use SHA-256 for everything else.
>>>> ---
>>>
>>> Please take some time to read through the comments that reviewers have left on
>>> previous versions of the patchset.
>>
>> So I went and read through the old comments, and I'm lost. In brief summary:
>>
>> If the hardware+firmware only supports SHA-1, then some reviewers would prefer
>> Linux not to support DRTM. I personally think this is a bit silly, but it's
>> not entirely unreasonable. Maybe it should be a config option?
>>
>> If the hardware+firmware does support SHA-256, then it sounds (to me, reading
>> this -- I haven't dug into the right spec pages) that, for optimal security,
>> something still needs to effectively turn SHA-1 *off* at runtime by capping
>> the event log properly. And that requires computing a SHA-1 hash. And, to be
>> clear, (a) this is only on systems that already support SHA-256 and that we
>> should support and (b) *not* doing so leaves us potentially more vulnerable to
>> SHA-1 attacks than doing so. And no SHA-256-supporting tooling will actually
>> be compromised by a SHA-1 compromise if we cap the event log.
>>
>> So is there a way forward? Just saying "read through the comments" seems like
>> a dead end.
>>
>
> It seems there may be a justification for some form of SHA-1 support in this
> feature. As I've said, the problem is that it's not explained in the patchset
> itself. Rather, it just talks about "SHA" and pretends like SHA-1 and SHA-2 are
> basically the same. In fact, SHA-1 differs drastically from SHA-2 in terms of
> security. SHA-1 support should be added in a separate patch, with a clearly
> explained rationale *in the patch itself* for the SHA-1 support *specifically*.

For the record, we were never trying to "pretend" or obfuscate the use
of SHA-1. It was simply expedient to put the hash algorithm changes in
one patch. We have now separated the patches for clarity and will add
any text that explains our use and/or explain the issues with its use.

We went back through the comments and tried to address everything that
came up about the use of SHA-1. We will review it all again before
posting another patch set.

Thank you for your feedback.
Ross

>
> - Eric